A night on the town wouldn't be complete without having a designated driver: one person who stays sober so other members of the group can get home safely.

A new report suggests that a designated driver isn't always as sober as people might expect.

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People who spoke to KMBC said the rules for what a designated driver should drink range from no more than one per hour to no drinks at all.

A new study published by the Journal of Studies on Alcohol and Drugs found 40 percent of designated drivers take a drink or two during the course of an event. Some designated drivers have even more.

KMBC took a Breathalyzer to a Kansas City-area bar and asked people to take the test. After recording a level below the legal limit for drunken driving, they said they were surprised to learn they were OK to drive.

Police said too many people fixate on the blood-alcohol number and get behind the wheel after having some drinks.

"I have arrested several people well below the point, then of .10 (percent) and now .08 (percent), who were obviously impaired," said Officer Andy Black of the Overland Park Police Department.

He said he's spent most of his 25-year career on drunken driving and traffic enforcement. He said the designated driver excuse doesn't always fly.

"I've probably only had eight or 10 in all the years I can remember who said they were the designated driver who had not been drinking some amount during the night," he said.

Simulator demonstrations show drivers can have trouble staying in their lanes at .01 percent, a level usually associated with one drink. At .02 percent, drivers can exhibit drowsiness. At .04 percent, vigilance and reasoning issues can occur. At .07 percent, reaction time and vision can take a hit.

One of the people who took KMBC's test, Scott Smith, said he has been pulled over in the past and was tested at .03 percent.

"It was a close call and now, I don't even mess with it," he said. "I just have one beer and that's it."

The legal blood-alcohol limit is .08 percent in Kansas and Missouri.

Black said officers are still allowed to make judgment calls and can arrest a driver for driving under the influence even if the driver tests below .08 percent.